Electroluminescent element



Dec. 13, 1960 H A KLASENS Ef AL 2,964,666

ELECTROLUMINESCENT ELEMENT Filed Sept. 19, 1957 FIGI F IG.2

lNvENTo Rs HENDRIK ANNE KLASENS AGE ELECTROLUMINESCENT ELEMENT Hendrik Anne Klasens and Hendrik Jacobus Maria Joormann, Eindhoven, Netherlands, assignors to North American Philips Company, Inc., New York, N.Y., a corporation of Delaware Filed Sept. 19, 1957, Ser. No. 685,038 Claims priority, application Netherlands Sept. 21, 1956 8 Claims. (Cl. 313-108) Our invention relates to electroluminescent elements and to improvements in such elements.

Electroluminescent elements comprising a substratum including an electrode, a layer of electroluminscent material dispersed in a suitable binder and applied to this substratum and an electrode covering this electroluminescent layer are well-known in the art. Usually in these prior art elements at least one of the electrodes is pervious to light produced in the electroluminescent layer. Although these elements produce fairly satisfactory results, the color gamut, i.e. the range of frequencies of the light produced is very limited and no intense electroluminescence is obtained at the longwave end of the spectrum.

A principal object therefore of our invention is to provide an electroluminescent element having superior properties.

A second principal object of our invention is to provide an electroluminescent element having an increased color gamut.

Still another object of our invention is to provide an electroluminescent element that has an improved light output particularly at the long-wave end of the spectrum.

These and other objects of our invention will appear as the specification progresses.

According to our invention we provide an electroluminescent element comprising a luminescent layer sandwiched between two electrodes transparent to the light emitted from said layer. The luminescent layer comprises a suspension or dispersion in a synthetic resin of a mixture of an electroluminescent material and a luminescent material that is excited by the radiation of the electroluminescent material to produce light of a longer wavelength. By means of our invention it is possible to produce light, the color of which is the result of the addition of the light color emitted by the electroluminescent material and the light emitted by the luminescent material. Further, if the electroluminescent material produces only ultraviolet rays or light which is completely converted by the luminescent material the color of the light emitted by the element is identical with the color of the light emitted by the luminescent material itself.

In any event, by the use of the electroluminescent elements of our invention, we are able to produce light of a far greater variety of frequencies, i.e. more colors and a higher light output particularly at the long-wave end of the spectrum.

In a preferred embodiment of our invention we provide a layer of reflective material between the luminescent layer and one of the electrodes. By means of such a reliective layer the light output through the electrode on the side of the luminescent layer remote from the retiective layer is greatly increased because the light radiated is the sum of the direct radiation from the side of the luminescent layer remote from the reflective layer and the reflected light. Titanium dioxide forms a particularly effective reflective layer because of its high dielectric constant and high reliective power. The rutile States atetltA ICC modification is preferred, particularly because of its superior electrical properties.

In preparing the reliective layer the titanium dioxide is dispersed in a synthetic resin binder preferably the same one used in the luminescent layer. It is also advantageous that the synthetic resin binder used in the reflective layer be mixed with the luminescent material used in the luminescent layer in the same concentration used in the reflective layer in order to prevent ditusion of the luminescent material from the luminescent layer into the reflective layer.

In preparing the luminescent layer of our invention we may use any electroluminescent material, suitable examples of which are sulfides or selenides of zinc or copper, activated if desired, with copper, silver, gold or manganese. Any organic luminescent material or pigment which is capable of converting the ultraviolet rays or light produced by the electroluminescent material into light of longer wave-length. Examples of suitable luminescent pigments which may be used are a manganese activated magnesium arsenate of a red luminescence, and zinc cadmium sulfide activated with copper of a orangered luminescence.

Examples of organic luminescent materials that may be used are fluorescein, rhodamine B, eosine, aeridine red, acriliavine and magdala red.

Among the many synthetic resins which can be used as binders are urea-formaldehyde resins, and epoxy resins such as the epoxy resin sold under the trade name Epon 1009. The latter is particularly useful when mixed with a small amount of melamine.

Our invention will now be described in greater detail with reference to the following examples and the accompanying drawing, Figure l of which shows a cross-section of an electro-luminescent element made in accordance with one embodiment of our invention and Figure 2 of which shows a second electroluminescent element made in accordance with another embodiment of our invention.

Example I 35 gs. of a copper activated zinc sulfide of a blue electro-luminescence were mixed with 29 gs. of a 65% solution of urea-formaldehyde in butanol. To this solution there was then added 8 gs. of a 2.5% solution of iiuorescein in a mixture of equal parts of glycol and ethanol. The resultant solution was then sprayed on the conductive side of a glass plate made conductive with a transparent layer of tin oxide in an amount sufficient to form a coating of about 40 microns thick. The thus coated glass plate was then heated in air at a temperature of about 1500" C. for about one hour. After cooling a layer of aluminum having a thickness of about 1 micron was formed by evaporation on the luminescent layer.

With about volts A.C., 50-60 c./sec. applied to the resultant element a light of almost uniform whiteness was produced.

Exmple II 35 gs. of a copper and aluminum activated zinc sulfide of a green eectroluminescence were mixed with 29 gs. of a 65% solution of urea-formaldehyde in butanol. To the resultant suspension there was added 6 gs. of a 5% solution of rhodam-ine B in equal parts of glycol and ethanol. The resultant suspension was then sprayed on the conductive side of a conductive glass plate similar t0 the one used in Example I in an amount such that a layer of about 90 microns thick formed after drying. The coated glass pate was then heated in a-ir at about C. for about half an hour. A hard luminescent layer was thus produced. A suspension prepared by mixing 35 gs. `of titanium dioxide of the rutile modification, 6 gs. of a 5% rhodamine B solution in equal parts of glycol and ethanol and 29 gs. of a 65 solution of urea-formaldehyde in butanol in a quantity such that after drying the thickness of the layer was 30 microns was then sprayed on the luminescent layer. The glass plate was then heated in air for about one hour at a temperature of about 155 C. Finally a silver layer of about 5 microns thick was applied by evaporation to the exposed surface of the rutile containing layer. With about 600 volts A.C., 50-60 c./sec. applied to this electroluminescent element a red electroluminescence was produced.

Example III 40 gs. of a copper activated Zinc sulfide of a blue luminescence was mixed with 60 gs. of a 65% solution of urea-formaldehyde in butanol and 30 gs. of a manganeseactivated magnesium arsenate of red luminescence. The resultant suspension was then sprayed on the conductive side of a glass plate similar to one used in Example I in such a manner that after drying the thickness of the resultant luminescent layer was 100 microns. The thus coated glass plate was then heated in air at a temperature of about 155 C. for about one hour. After cooling, a reflective layer of white rutile of about 30 microns thick was applied to the luminescent layer by spraying the luminescent layer with a dispersion of rutile in a solution of urea-formaldehyde in butanol and then heating the glass plate in air at a temperature of about 155 C. for about one hour. After cooling, a silver layer of about microns thick was applied on the rutile layer by spraymg.

With about 300 volts A.C., 50-60 c./sec. applied to the resultant luminescent element a purple color was produced.

Fig. l illustrates a luminescent element made in accordance with Example I. In this figure a glass plate 1 has a thin transparent electrically conducting coating 2 of tin oxide. On the coating 2 there is a luminescent layer 3, about 40 microns thick produced by the method of Example I and on top of the luminescent layer at the side away from the glass there is a layer of aluminum 4 about 1 micron thick.

Figure 2 illustrates a luminescent element made in accordance with Example II. In this ligure a glass plate 1 has thin transparent electrically conducting coating of tin oxide. On the coating 2 there is a luminescent layer 5 of about 90 microns made in accordance with Example II. On the luminescent layer 5 there is a reective layer 6, 30 microns thick, of titanium dioxide and rhodamine B in a urea-formaldehyde resin binder made in accordance with Example II. On the reective layer 6 there is a thin layer 7 of silver of about 5 microns thick.

While we have described our invention in connection with specific embodiments and applications, other modications thereof will be readily apparent to those skilled in this art without departing from the spirit and scope of the invention as deined in the appended claims.

What is claimed is:

l. An electroluminescent element comprising a luminescent layer sandwiched between two electrodes, said l-ayer comprising a mixture of an electroluminescent material and a luminescent material, activated to luminescence by the radiation from the electroluminescent materi-al, dispersed in a synthetic resin.

2. An electroluminescent element comprising a luminescent layer sandwiched between two electrodes, one of said electrodes being transparent to luminescence from the luminescent layer, said layer comprising a mixture of an electro-luminescent material and a luminescent material, activated to luminescence by the radiation from the electro-luminescent material, dispersed in a synthetic resin.

3. An electroluminescent element comprising a luminescent layer sandwiched between two electrodes, one of said electrodes being transparent to luminescence from the luminescent layer, said layer comprising a mixture of an electroluminescent material and a luminescent material, activated to luminescence by the radiation from the electroluminescent material, dispersed in a synthetic resin, and a layer of titanium-dioxide dispersed in a synthetic resin and positioned between the luminescent layer and the electrode other than the electrode transparent to luminescence.

4. An elecro-luminescent element comprising a luminescent layer sandwiched between two electrodes, one of said electrodes being transparent to luminescence from the luminescent layer, said layer comprising a mixture of an electroluminescent material and an organic luminescent material soluble in the synthetic resin, activated to luminescence by the radiation from the electroluminescent material, dispersed in a synthetic resin, and a layer of titanium-dioxide dispersed in a synthetic resin a-nd positioned between the luminescent layer and the electrode other than the electrode transparent to luminescence.

5. An electroluminescent element comprising a luminescent layer sandwiched between two electrodes, one of said electrodes being transparent to luminescence from the luminescent layer, said layer comprising a mixture of an electroluminescent material soluble in the synthetc resin, activated to luminescence by the radiation from the electroluminescent material, dispersed in a synthetic resin, and a layer of titanium dioxide dispersed in said synthetic resin and positioned between the luminescent layer and the electrode other than the electrode transparent to luminescence.

6. An electroluminescent element comprising a luminescent layer sandwiched between two electrodes, one of said electrodes being transparent to luminescence from the luminescent layer, said layer comprising a mixture of an electroluminescent material and an organic luminescent material soluble in the synthetic resin, activated to luminescence by the radiation from the electroluminescent material, dispersed in a synthetic resin and a layer of titanium dioxide and said organic luminescent material in the same concentration as present in the luminescent layer dispersed in said synthetic resin and positioned between the luminescent layer and the electrode other than the electrode transparent to luminescence.

7. The electroluminescent element of claim 6 in which the synthetic resin is a urea-formaldehyde resin.

8. The electroluminescent element of claim 6 in which the synthetic resin is an epoxy resin.

References Cited in the le of thispatent UNITED STATES PATENTS 2,413,940 Bickford Jan. 7, 1947 2,494,883 Kroger et al. Jan. 17, 1950 2,624,857 Mager Jan. 6, 1953 2,692,349 Ouweltjes Oct. 19, 1954 2,728,870 Gungle et al Dec. 27, 1955 2,774,903 Burns Dec. 18, 1956 2,783,407 Vierkotter Feb. 26, 1957 FOREIGN PATENTS 204,854 Australia May 26, 1955 OTHER REFERENCES Luminesoence of Solids, H. W. Leverenz, John Wiley and Sons, New York, N.Y., 1950, pages 411, 412 and 470.

Crystal Ball Plots 3-D Curves In Color, by I. R. Alburger, Electronic Industries & Tele-Tech, February 1957, pages 50 to 53.

Electroluminescent Cell With Long Light-Decay and Color Shift, by Simon Larach, R.C.A. TN, No. 10, Aug. 9, 1957.

Electroluminescence With Storagej by Simon Larach, R.C.A. TN, No. 18, Aug. 9, 1957. 

